Polyphase filter with integrators

ABSTRACT

Polyphase filters comprising filters each filter comprising passive elements. The filters are provided with integrators comprising amplifiers with admittance elements in feedback paths to create one or more other poles not situated on the negative imaginary axis in the plane pole-zero plot. A conductance element couples an output of an integrator to an input of a previous integrator for introducing a frequency shift for at least one pole in the plane pole-zero plot. A capacitor couples an output of an integrator to an input of a next integrator for improving the quality factor of the polyphase filter, which factor can then exceed. A signal inversion allows conductance elements to have negative values necessary for locating at least one pole at the most optimal location in the plane pole-zero plot.

The invention relates to a polyphase filter comprising at least twofilters for filtering signals.

The invention also relates to an integrator for use in a polyphasefilter comprising at least two filters for filtering signals, and to areceiver comprising a polyphase filter comprising at least two filtersfor filtering signals.

Such a polyphase filter is for example a two-phase filter comprising twofilters for filtering two (input) signals, or a four-phase filtercomprising four filters for filtering four (input) signals. Polyphasefilters are used, for example, in television receivers, for example inthe tuner just before the mixer.

A prior art polyphase filter is known from U.S. Pat. No. 3,559,042,published Jan. 26, 1971, which discloses a polyphase network including Nsingle phase circuits (filters), each circuit having a first impedancecoupled between input and output terminals of said circuit, with saidinput terminal also being coupled to the output terminal of anothercircuit responding to an adjacent phase (leading or lagging) of theinput signal by a second impedance having a different phase anglecharacteristic than that of the first impedance.

The known polyphase filter is disadvantageous, inter alia, due to, forexample in case of being used in a tuner just before the mixer, needingbuffers, said buffers limiting the dynamic range of the polyphasefilter.

It is an object of the invention, inter alia, of providing a polyphasefilter as defined in the preamble which allows the dynamic range to belarger.

The polyphase filter according to the invention is characterized in thatsaid filters are coupled to integrators for integrating filteredsignals.

The integrators coupled to said filters allow a larger dynamic rangethan the known combination of filters and buffers.

The invention is based upon an insight, inter alia, that a knownpolyphase filter with buffers has a limited dynamic range, and is basedupon a basic idea, inter alia, that integrators replacing said buffersallow the dynamic range to be larger.

The invention solves the problem, inter alia, of providing a polyphasefilter as defined in the preamble which allows the dynamic range to belarger, and introduces further advantageous functions, as describedbelow.

According to a first embodiment of the polyphase filter according to theinvention, an output of an integrator is coupled via a conductanceelement to an input of a previous integrator.

Said conductance element increases the selectivity of the polyphasefilter. In case of a two-phase filter, generally there will be twofilters each with an integrator. Then the output of the secondintegrator is coupled via a first conductance element to the input ofthe first integrator, and generally the output of the first integratoris coupled via a second conductance element to the input of the secondintegrator. In case of a four-phase filter, generally there will be fourfilters each with an integrator. The output of the second integrator iscoupled via a first conductance element to the input of the firstintegrator, and generally the output of the third integrator is coupledvia a second conductance element to the input of the second integrator,etc.

According to a second embodiment of the polyphase filter according tothe invention an output of an integrator is coupled via a capacitor toan input of a next integrator.

Said capacitor further increases the selectivity of the polyphasefilter. In case of a two-phase filter, the output of the firstintegrator is coupled via a first capacitor to the input of the secondintegrator, and generally the output of the second integrator is coupledvia a second capacitor to the input of the first integrator. In case ofa four-phase filter, the output of the first integrator is coupled via afirst capacitor to the input of the second integrator, and generally theoutput of the second integrator is coupled via a second capacitor to theinput of the third integrator, etc. Said capacitor improves the qualityfactor of the polyphase filter, allowing a quality factor larger than½{square root}{square root over (2)} without making a loop-amplificationlarger than one in a three-phase, four-phase, five-phase etc. filter.

According to a third embodiment of the polyphase filter according to theinvention an integrator comprises an amplifier with an admittanceelement in a feedback path.

Said integrator is provided with an amplifying function and can beintegrated well in an integrated circuit, with said admittance elementdefining an integration factor.

According to a fourth embodiment of the polyphase filter according tothe invention a filter comprises a passive element and an amplifiercomprises an operational amplifier.

This polyphase filter based on filters each comprising passive elementsand operational amplifiers can be integrated well in an integratedcircuit, with each part of said fourth embodiment being a sub-embodimentpossibly to be used in combination with one or more othersub-embodiments or not.

According to a fifth embodiment of the polyphase filter according to theinvention a passive element comprises a resistor and a capacitor and anadmittance element comprises a capacitor and a conductance elementcoupled in parallel to each other.

This polyphase filter based upon resistors and capacitors and admittanceelements comprising capacitors and conductance elements coupled inparallel to each other can be integrated well in an integrated circuit,with each part of said fifth embodiment being a sub-embodiment possiblyto be used in combination with one or more other sub-embodiments or not.

According to a sixth embodiment of the polyphase filter according to theinvention said polyphase filter comprises at least one signal inversionbetween integrators.

Said signal inversion allows a conductance element to have a negativevalue necessary for locating at least one pole at the most optimallocation in the plane pole-zero plot. A signal inversion may be realizedby exchanging a pair of connections in an integrated circuit (exchangingplus and minus) in case of a balanced polyphase filter or may berealized by an inverter for example comprising an operational amplifier.

It should be noted that the words “previous” and “next” should not belooked at too narrowly and that these words may correspond with“neighboring”. Between an integrator and a previous/next/neighboringintegrator, there could be located further integrators or not. Usually,but not exclusively, said integrators will be coupled more or lesssymmetrically.

Embodiments of the integrator according to the invention and of thereceiver according to the invention correspond with the embodiments ofthe polyphase filter according to the invention.

These and other aspects of the invention will be apparent from andelucidated with reference to the embodiments(s) described hereinafter.

FIG. 1 illustrates in block diagram form a polyphase filter according tothe invention comprising four filters and four integrators according tothe invention, and

FIG. 2 illustrates in block diagram form a polyphase filter according tothe invention comprising two filters and two integrators according tothe invention with an inverter.

The polyphase filter shown in FIG. 1 comprises four filters 1-4. Filter1 (2,3,4) comprises a capacitor 10 (20,30,40) located between an inputand an output of filter 1 (2,3,4), with a resistor 11 (21,31,41) beingcoupled to the input of filter 1 (2,3,4) and to an output of filter 2(3,4,1). The output of filter 1 (2,3,4) is coupled to a first input ofan operational amplifier 12 (22,32,42), of which a second input iscoupled to ground and of which an output is coupled via an admittanceelement 13 (23,33,43) to said first input. Said output is furthercoupled via a conductance element 44 (14,24,34) to an input of anoperational amplifier 42 (12,22,32), and is further coupled via acapacitor 25 (35,45,15) to an input of an operational amplifier 22(32,42,12).

The output of first (second, third, fourth) filter 1 (2,3,4) is coupledto an input of a first (second, third, fourth) integrator 12-13(22-23,32-33,42-43) comprising an operational amplifier 12 (22,32,42)with an admittance element 13 (23,33,43) in a feedback path. Otheramplifiers like for example a single or a few transistors are howevernot to be excluded. Said admittance element 13 (23,33,43) for examplecomprises a capacitor and a conductance element coupled in parallel toeach other, but other and/or further admittance elements belong to thepossibilities.

The output of first (second, third, fourth) integrator 12-13(22-23,32-33,42-43) is coupled via a conductance element 44 (14,24,34)to an input of a previous integrator 42-43 (12-13,22-23,32-33), withfurther elements not to be excluded.

The output of first (second, third, fourth) integrator 12-13(22-23,32-33,42-43) is coupled via a capacitor 25 (35,45,15) to an inputof a next integrator 22-23 (32-33,42-43,12-13), with further elementsnot to be excluded.

The first (second, third, fourth) filter 1 (2,3,4) comprises passiveelements like said capacitor 10 (20,30,40) and said resistor 11(21,31,41), with other and/or further passive elements not to beexcluded. Usually, at least two passive elements in each filter 1,2,3,4will have a different phase angle characteristic.

Alternatively, instead of a filter 1 (2,3,4) comprising passive elements10-11 (20-21,30-31,40-41) and being coupled to an integrator 12-13(22-23,32-33,42-43), a filter 1 (2,3,4) may comprise passive elements10-11 (20-21,30-31,40-41) and may further comprise an integrator 12-13(22-23,32-33,42-43) and/or a conductance element (thereby having theoption of including either the one coupled to the previous integrator orthe one coupled to the next integrator) and/or a capacitor (and therebyagain having the option of including either the one coupled to theprevious integrator or the one coupled to the next integrator).

Amplifiers with admittance elements in feedback paths introduce poles onthe negative real axis in the plane pole-zero plot. The conductanceelements introduce frequency shifts of said poles in the plane pole-zeroplot. The capacitors also give frequency shifts of said poles in theplane pole-zero plot. Quality factors larger than ½{square root}{squareroot over (2)} in a four-phase filter now belong to the possibilities.

The polyphase filter shown in FIG. 2 comprises a first (second)operational amplifier 50 (55) of which a first input is coupled to anadmittance element 52 (57) and of which a second input is coupled toground and of which an output is coupled via an admittance element 51(56) to said first input. Another side of admittance element 52 (57)forms a first (second) input of this polyphase filter. Said output offirst (second) operational amplifier 50 (55) forms a first (second)output of this polyphase filter.

Said first input of this polyphase filter is further coupled to aconductance element 58. Another side of the conductance element 58 iscoupled to a first input of a third operational amplifier 60. A secondinput of the third operational amplifier 60 is coupled to ground. Anoutput of the third operational amplifier 60 is coupled via aconductance element 61 to said first input of third operationalamplifier 60 and via a conductance element 62 to said first input ofsecond operational amplifier 55. Said first input of third operationalamplifier 60 is further coupled via a conductance element 59 to theoutput of first operational amplifier 50.

Said second input of this polyphase filter is further coupled to aconductance element 53, of which an other side is coupled to said firstinput of first operational amplifier 50 and via a conductance element 54to said output of second operational amplifier 55.

Admittance element 52 and conductance element 58 form a first filter,with operational amplifier 50 with admittance element 51 forming a firstintegrator. Admittance element 57 and conductance element 53 form asecond filter, with operational amplifier 55 with admittance element 56forming a second integrator. Conductance element 54 (59) couples anoutput of a second (first) integrator to an input of a first (second)integrator.

Third operational amplifier 60 together with conductance elements 61 and62 forms an inverter located between both integrators and allowsconductance elements 58 and 59 to have a negative value necessary forlocating at least one pole at the most optimal location in the planepole-zero plot.

With respect alternatives and/or additions to said operationalamplifiers, admittance elements and conductance elements, see above.

Let us consider that the admittance elements 51 and 56 areY_(b)=sC_(b)+G_(br), admittance elements 52 and 57 areY_(a)=sC_(a)+G_(ar), conductance element 53 is G_(a) and conductanceelement 58 is −G_(a), conductance element 54 being G_(b) and conductanceelement 59 being −G_(b). Said negative values are realized via saidinverter. The transfer function becomes:Hp(s)=−(sC _(a) +G _(ar) +jG _(a))/(sC _(b) +G _(br) +jG _(b)),

-   -   and a pole can be found at s_(x)=−(G_(br)+jG_(b))/C_(b),    -   and a zero can be found at s_(o)=−(G_(ar)+jG_(a))/C_(a).

It is remarked that the scope of protection of the invention is notrestricted to the embodiments described herein. Neither is the scope ofprotection of the invention restricted by the reference numerals in theclaims. The word ‘comprising’ does not exclude other parts than thosementioned in the claims. The word ‘a(n)’ preceding an element does notexclude a plurality of those elements. Means forming part of theinvention may both be implemented in the form of dedicated hardware orin the form of a programmed purpose processor. The invention resides ineach new feature or combination of features.

1. Polyphase filter comprising at least two filters for filteringsignals, characterized in that said filters are coupled to integratorsfor integrating filtered signals.
 2. Polyphase filter according to claim1, wherein an output of an integrator is coupled via a conductanceelement to an input of a previous integrator.
 3. Polyphase filteraccording to claim 2, wherein an output of an integrator is coupled viaa capacitor to an input of a next integrator.
 4. Polyphase filteraccording to claim 3, wherein an integrator comprises an amplifier withan admittance element in a feedback path.
 5. Polyphase filter accordingto claim 4, wherein a filter comprises a passive element and wherein anamplifier comprises an operational amplifier.
 6. Polyphase filteraccording to claim 5, wherein a passive element comprises a resistor anda capacitor and wherein an admittance element comprises a capacitor anda conductance element coupled in parallel to each other.
 7. Polyphasefilter according to claim 6, wherein said polyphase filter comprises atleast one signal inversion between integrators.
 8. Integrator for use ina polyphase filter comprising at least two filters for filteringsignals, wherein said filters are coupled to integrators for integratingfiltered signals.
 9. Receiver comprising a polyphase filter comprisingat least two filters for filtering signals, wherein said filters arecoupled to integrators for integrating filtered signals.